20 MISC. PUBLICATION 3 4 0, IT. S. DEPT. OF AGKICULTUKE 



bitter taste. The tissues, however, become tough and leathery, so 

 that the softening never even approximates the mushy, leaky condition 

 produced by Rhizopus, nor the soft-spot condition produced by certain 

 other molds. The discoloration, particularly on the outside, varies so 

 greatly that it must be described in detail. 



In the field, berries may be affected at all stages from blossoming to 

 full maturity; the picture presented by each stage, if these are not 

 chosen too close together, has characteristics which set it off from that 

 presented by each of the others. In fruits that have barely begun to 

 enlarge after petal fall, ail parts become dark brown over affected 

 areas, shading off into the natural green of the unaffected area around 

 them; fruits mature enough to have turned red show, over affected 

 areas, the same yellow to light brown at the center of the spot, but with 

 a transition from this through darker brown to purple to the natural 

 red of the berry; ripe, fully colored fruits sometimes show no color 

 change at all except a slight darkening of the red over affected spots, 

 or sometimes a faint tinge of the purple which forms the transition to 

 red on less mature fruits (pi. 9, B). 



On the market only the most advanced stages are seen. Here, 

 however, a superficial growth of white mold, rare in the field, is 

 frequently seen, and will serve as a diagnostic characteristic if taken 

 in connection with the external color changes and the internal 

 appearance. 



In cross and longitudinal sections, strawberries affected by leather 

 rot show a marked browning of the water-conducting system, accom- 

 panied usually by a less intense browning of all the other tissues (pi. 

 9, (7). In very early stages this browning in the vessels is often the 

 only visible symptom. So far as can be noticed by sight or by touch, 

 affected tissues are not disintegrated, though they are always softened 

 and become decidedly tough and leathery. At no time is there any 

 clear line of demarcation between diseased and healthy flesh, nor can 

 a separation of the two be made by mechanical means; it is not pos- 

 sible to lift or scoop out the part affected as can so easily be done 

 with the rotten spots produced by Rhizopus. 



CAUSAL FACTORS 



Leather rot is caused by the fungus Phytophthora cactorum,, an 

 active parasite which is able to penetrate the uninjured skin of straw- 

 berry fruits. The disease has always been found associated with wet 

 weather in the field and for that reason is known to growers as water 

 soak. This use of the term implies a belief, often explicitly stated, 

 that the berries take up too much water and become worthless. This 

 probably does happen with some of the fruit during wet weather, for it 

 is well known that strawberries produced during such weather are 

 likely to be soft, succulent, and easily damaged even by the most care- 

 ful handling. Moderately high temperatures are important also. In 

 White County, Ark., during one rainy season when every rain was 

 followed by a cool spell, leather rot was practically negligible, whereas 

 during the corresponding season of the year before, also wet but much 

 warmer, the rot cut down the marketable crop by fully 20 percent. 



The factor of time must also be considered in dealing with this rot. 

 Field observations have shown that with warm weather following heavy 

 rains leather rot does not appear until about the third day after the 



